The invention concerns a process and a device for producing silicon carbide monocrystals.
For sublimation growing of silicon carbide monocrystals from silicon carbide (SiC), a certain amount of silicon carbide, usually in powdered form or in the form of a pre-manufactured solid SiC crystal, is heated and at least partially sublimated in a storage area. The sublimated SiC is made to grow on a SiC seed crystal located in a reaction chamber. Due to the high temperatures required for sublimation growing, in general over 2000.degree. C., the walls surrounding the storage area and the reaction chamber are usually made of refractory materials such as graphite or metal carbides. However, as it travels between the storage area and the seed crystal, sublimated SiC in the gaseous phase unavoidably comes into contact with the container walls. This may cause the impurities contained in the materials of the container walls to be removed by the silicon-rich SiC gaseous phase and end up in the growing SiC monocrystal. Furthermore, the stoichiometric composition of the SiC in the gaseous phase can change through chemical reactions of silicon in the SiC gaseous phase with the graphite or metal carbide walls. This can result in carbon-rich deposits on the growing SiC monocrystal.
In the Lely sublimation process known from U.S. Pat. No. 2,854,364 and AT-PS-243 for growing SiC monocrystals, fragments of SiC monocrystals are stratified in a graphite container so that they enclose a cavity (sublimation area). At a temperature of approximately 2500.degree. C., the SiC fragments are fused together and small SiC crystals are formed from the fused fragments. In this known process, the sublimation area is not hermetically enclosed. In particular, doping gases are passed in the spaces between the SiC fragments to dope the growing SiC crystals.
German Offenlegungsschrift 24 09 005 and the corresponding U.S. Pat. No. 4,147,572 disclose a process and a device for epitaxial manufacturing of monocrystalline SiC layers consisting of SiC seed crystals, wherein a polycrystalline SiC plate is arranged as a vapor source at a distance not exceeding twice the linear dimension of the SiC plate consisting of a plurality of SiC seed crystals. A graphite stop ring, mounted on a graphite support for the seed crystals, on whose opposite side the SiC plate is arranged, is used as a spacer. One or more such growth cells comprising graphite support, seed crystals, graphite stop ring, and SiC plate, are mounted in a graphite crucible, which is placed in a furnace. The sublimation process is carried out in the furnace at a pressure of approximately 1bar to 10.sup.-5 Torr and at temperatures between 1600.degree. C. and 2400.degree. C. Doping gases are introduced in the graphite crucible through an opening to dope the SiC epitaxial layers.
Japanese Patent A-04-055397 discloses the use of polycrystalline .beta.-SiC produced by chemical vapor deposition (CVD) for sublimation growing of SiC monocrystals.
In another device, known from U.S. Pat. No. 5,288,326 for producing SiC monocrystals, solid SiC particles are obtained in a reaction chamber from a gas mixture containing silane, propane, and hydrogen as main components, which drop into a sublimation chamber and are evaporated there at temperatures between 2000.degree. C. and 2400.degree. C. The sublimated SiC is grown on a seed crystal.